As known, civil transport airplanes should be pressurized, as upon a cruise flight, an airplane flies at an altitude being often higher than 30,000 feet (about 9,000 meters), for which the external air is too low in oxygen (and also too cold and too dry) for being compatible with life. Thus, pressurizing systems are provided in airplanes for keeping on board a breathable atmosphere. In particular, the international aeronautic regulation states that any public transport airplane flying at an altitude higher than 20,000 feet (about 6,000 meters) should be pressurized and that it should establish in the cockpit an equivalent altitude which does not exceed 8,000 feet (about 2,400 meters) upon a normal flight.
It may however occur that, as a result of a breakdown or an incident, the pressurization in the airplane could no longer be maintained at an acceptable level. A regulatory procedure then compels the pilot to have the airplane descent, as quickly as possible, at a breathable altitude of 10,000 feet (about 3,000 meters) or at the current security altitude if it is not possible to descent as low as 10,000 feet because of the relief. Such a procedure is referred to as an emergency descent.
In such a case, the crew is responsible for different tasks related to initiating the descent, as well as the adjustment of parameters of the descent (speed, target altitude, lateral trajectory, etc.) and this until the airplane flies level at low altitude.
It could happen, however, although very rarely, that the crew is no longer able to apply the above described procedure, for instance in the case of a pressurization breakdown as a result of which the crew have lost conscience. The airplane is, in such a case, unattended, while it is absolutely necessary to carry out an emergency descent. If, in such a situation, the autopilot is activated, the flight is continued automatically until the kerosene supplies are totally exhausted.
In order to avoid such a situation, an autopilot system is known, allowing, when it is engaged, to carry out the emergency descent automatically, that is without requiring the help of a pilot. Moreover, engaging such an automatic emergency descent could be carried out either manually by the pilot, or also automatically.
In particular, from document FR 2,928,465, a particular method is known for automatically controlling an emergency descent of an aircraft. According to this method, when an emergency descent automatic function is engaged, the following successive operations are carried out:
a) a set of vertical setpoints is automatically determined, comprising:                a target altitude representing an altitude to be reached by the aircraft at the end of the emergency descent; and        a target speed representing a speed that the aircraft should respect upon the emergency descent;        
b) a set of lateral setpoints is automatically determined, representing a lateral maneuver to be carried out upon the emergency descent; and
c) the aircraft is automatically guided so that it simultaneously respects said set of vertical setpoints and said set of lateral setpoints until reaching said target altitude that it subsequently maintains, said automatic guidance being able to be interrupted by an action of the pilot of the aircraft.
Furthermore, this known method provides particular means for automatically engaging the emergency descent function, taking into account the variation of altitude of the cockpit, that is the variation of pressure inside the cockpit.
As far as the determination of a target altitude is concerned within the context of an automated emergency descent, the following is known:                from document U.S. Pat. No. 4,314,341, an automated emergency descent to a security altitude, the value of which is inclusively fixed to 2000 feet (about 3600 m). Such a value corresponds to a physiologically breathable and satisfactory altitude, but it could be lower at the highest grounds (Alpes, Himalayas, Andes, Rocky Mountains, etc.). Thus, it is not satisfactory to ensure a secured end of maneuver, should a crew be unconscious (possible collision with the ground);        from U.S. Pat. No. 6,507,776, a coupling between an autopilot and a GPS system having a data base wherein values of altitude are stored for all the reliefs, having the altitude higher than or equal to a fixed maximum value. Such a GPS system is provided with a device for identifying the relief along the current trajectory. Such a device allows the autopilot to be provided with the lowest possible security target altitude, available by adjusting the heading of the aircraft if needed, for bypassing the ground. Such a device has the drawback of directing the aircraft outside the area covered by the initially followed air traffic lane. The associated risk involves increasing the workload of the crew when they are conscious again, as the aircraft is likely to fly far from the initially followed flight itinerary, and, moreover, not to have enough kerosene available for reaching the closest deviating airport,        from document US 2007/0,043,482, another device integrated into an autopilot able to carry out automatically an emergency descent to a security altitude, the calculation thereof being based on security minimum altitudes of the MSA type (<<Minimum Safe Altitude>>). More precisely, a data base containing the MSA altitudes is used for determining the associated security altitude, either at the current flight itinerary, or, should it exist, at a deviation trajectory provided by the airline company. When the airplane is outside the flight itinerary or outside a deviation lane, the security altitude is calculated from the data base of the ground, taking as a value, the maximum altitude on a trajectory maintaining the current heading. Such a device taking into consideration the surrounding relief has the drawback of targeting a potentially too high and therefore, inappropriate altitude for allowing an unconscious crew to be able to keep the airplane under control again. Indeed, the risk involves hindering the descent strategy, selecting a security altitude corresponding to a point located much ahead the current trajectory of the airplane or the current flight itinerary, whereas a lower descent could be contemplated while leaving enough downstream margin, to adjust the deviation.        
Consequently, none of such usual solutions is completely satisfactory, as none of them allow to provide, in all circumstances, an optimum target altitude value that, both, takes into account the surrounding relief for avoiding to descent the airplane at an altitude likely to cause a collision with the ground, and is the lowest possible to as to maximize the chances allowing a crew to keep the airplane under control again.